The conventional filters for air cleaners include a filter which is made by compressing the conically-molded filter paper from the above in a plurality of steps into a concentrically-folded structure, or folding the flat filter paper in the shape of the teeth of a saw as shown in FIGS. 6 and 7, and which is set in a filter body having air inlet and outlet ports. However, this filter for air cleaners has the following drawbacks. The area of the filtering surface of the filter cannot be increased greatly in comparison with the cross-sectional area of an air-passing portion thereof. The impact strength of the filter medium is low, and the steps of manufacturing the filter medium are complicated. The cost of manufacturing the filter medium is high.
An object of the present invention is to provide a filter for air cleaners, having a large effective area in comparison with the cross-sectional area of an air-passing portion thereof, and a high impact strength.